1. Field of the Invention
This invention relates in general to a bioelectrical impedance measurement apparatus.
2. Description of the Related Art
Bioelectrical impedance measurements have been employed to determine various body characteristics, such as blood flow, cardiac output and composition including an assessment of body fat, lean body mass and body cell mass. To determine body composition, a four-electrode impedance plethysmograph is usually employed. A first pair of source or current electrodes is connected to a human body typically on a hand and a foot. Specifically, one source electrode is attached to the dorsal surface of a hand over the metacarpals, whereas the other source electrode is attached to the distal end of the third metatarsal bone of the foot. One electrode of a second pair of detecting or sensing electrodes is traditionally attached to the dorsal surface of a hand between the bony prominences of the wrist, whereas a second sensing electrode is positioned between the lateral and medial bony prominence of an ankle. An excitation current generated by the plethysmograph is applied to the source electrodes and thus introduced into the body. For example, an 800 microampere, 50 kHz current is typically employed.
The human body opposes the conduction of electrical current, and this ability to oppose current is called impedance. Impedance is measured by the plethysmograph, which generally includes a measuring circuit, an amplifier and an indicator circuit. Then, the impedance is used to predict other physiological parameters. For example, body resistance can be combined in an equation with the weight and height of the subject to predict total body water (TBW).
Existing measuring devices do not isolate the subject from the electronics of the measuring device to prevent potential electrical shocks and improve the common mode rejection ratio. Further, accuracy of measurement is important because of the use of the measurements as predictors of health. Many of the inaccuracies of the impedance measurements are related to noise generated by the existing measuring circuits themselves. The more noise generated, the less likely small changes in impedance are able to be measured. Temperature sensitivity can also be a problem in existing devices, particularly when the device is portable.
It would be desirable to provide a bioelectrical impedance measurement apparatus that increases the accuracy of a single body compositional measurement of a subject while isolating the subject from the electronic components of the apparatus. It would also be desirable to make the apparatus portable.
The present invention is an apparatus for determining bioelectrical impedance measurements. Specifically, the present invention is an apparatus for measuring an impedance of a segment of a subject, comprising: a constant current source wherein a level of an input current is controlled by a feedback loop using an error signal representing a difference between an actual current measured through the segment and a target current; and an impedance measuring circuit detecting an output voltage across the segment and adapted to produce at least one of a reactance output signal and a resistance output signal using the output voltage.
In one aspect of the invention, the constant current source comprises: a direct current power supply providing a voltage source to an oscillating current source; a first amplifier connected to the oscillating current source wherein the gain of the first amplifier is controlled by the feedback loop; and an input coupling transformer receiving the output of the first amplifier and adapted to supply the input current through first external leads connectable to the segment. In a preferred aspect, the oscillating current source is a Colpitts oscillator.
In another aspect of the invention, the feedback loop of the constant current source comprises: a reference resistor coupled to a reference coupling transformer for measuring a constant current signal corresponding to the actual current through the segment; means for producing the error signal by comparing the constant current signal to a reference signal corresponding to the target current; and a modulator for controlling the gain of the first amplifier using the error signal. In a preferred aspect, the modulator is an optocoupler that changes resistance based on a value of the error signal.
In another aspect of the invention, the means for producing the error signal comprises: a buffer amplifier connected to the reference transformer and adapted to produce an amplified constant current signal; a rectifier connected to the buffer amplifier and adapted to rectify the amplified constant current signal; and a comparator receiving an output of the rectifier and the reference signal and producing the error signal. The rectifier can be a precision rectifier.
In yet another aspect of the present invention, the impedance measuring circuit comprises: a detection coupling transformer adapted to detect the output voltage; an output amplifier connected to the detection transformer adapted to produce an amplified output signal; and means for converting the amplified output signal into the reactance output signal and the resistance output signal. The amplified output signal can be capacitively coupled to the means for converting the amplified output signal into the reactance output signal and the resistance output signal. Preferably, the detection coupling transformer is designed for a common mode rejection ratio of greater than 90 dB at its operating frequency.
In one aspect of the invention including the means for converting the amplified output signal into the reactance output signal and the resistance output signal, the means includes: an integrator connected to receive the amplified constant current signal; a first balanced synchronous demodulator adapted to produce a first and a second direct current differential signal using a first reference vector provided by the integrator; a second balanced synchronous demodulator adapted to produce a third and a fourth direct current differential signal using a second reference vector provided by the buffer amplifier; a first instrumentation amplifier adapted to convert the first and second direct current differential signals into the reactance output signal; and a second instrumentation amplifier adapted to convert the third and fourth direct current differential signals into the resistance output signal. Preferably, the first and the second balanced synchronous demodulators are two passive analogue phase detectors.
In yet another aspect of the invention, the invention further comprises an automatic shut down circuit adapted to shut down operation of at least one of the constant current source and the impedance measuring circuit after at least one predetermined time period. The automatic shut down circuit can include isolating means to isolate a power supply from at least one of the constant current source and the impedance measuring circuit.
The invention can further comprise reporting means for reporting at least one of the reactance output signal and the resistance output signal.
In one aspect of the invention, the constant current source supplies the input current through first external leads connectable to the segment and the impedance measuring circuit measures the output voltage through second external leads connectable to the segment.
Another aspect of the apparatus for measuring an impedance of a segment of a subject comprises: a constant current source supplying a level of an input current to an input coupling transformer through first external leads connectable to the segment; a detection coupling transformer using second external leads connectable to the segment, which transformer detects an output voltage across the segment; and a circuit adapted to produce at least one output signal using the output voltage.
In one aspect, the invention further comprises a reference resistor coupled to a reference coupling transformer for measuring a constant current signal corresponding to an actual current through the segment and providing an input to the constant current source.
In another aspect of the invention, the level of the input current is controlled by an error signal representing a difference between an actual current measured through the segment and a target current. Preferably, an optocoupler controls the level of the input current using the error signal.
In yet another aspect, the detection coupling transformer is designed for a common mode rejection ratio of greater than 90 dB at its operating frequency.
Thus, the present invention is an apparatus that measures the resistance and reactance of a subject, or portion of a subject, directly while isolating the subject from the electronic components of the apparatus. The invention further provides a very accurate body composition measurement due to low noise. The extremely low power consumption of the apparatus and its relative temperature insensitivity contribute to its easy portability.